Make the URI of Servers and Load Balancers available.

The URIs are needed when working with the Rackspace Cloud Reverse DNS API, see http://docs.rackspace.com/cdns/api/v1.0/cdns-devguide/content/ReverseDNS-123456999.html

If we make them available in Servers and LBs, users can easily get a handle on them and pass them to the Reverse DNS API.

PR for Reverse DNS API to follow shortly.

Will want to cherry pick this into 1.6.x.